The Syrian Network for Human Rights, based in London, made the charges in a report released Saturday. The report followed by two days a U.S. intelligence assessment alleging forces loyal to Syrian President Bashar Assad had unleashed chemical weapons, killing 100 to 150 people.
SNHR said it had confirmed the attacks through testimony from survivors, witnesses and doctors in field hospitals.
The team that worked to document the attacks "encountered great difficulties" in gathering and analyzing information, SNHR said, because the Syrian government prevented the group from working freely and chased its members.
The organization said it could find no instances in which rebel forces used chemical weapons or poisonous gases.
SNHR said it documented attacks in five governorates, or states, beginning in December. The most recent incident documented occurred June 9 in the countryside around Damascus in which regime forces shelled opposition Free Syrian Army troops with poisonous gases. Four people were treated for breathing problems.
The most deadly attack took place March 19, when SNHR said Syrian warplanes dropped missiles carrying chemical warheads on the community of Khan Alasa'l in Aleppo, killing 22 people and injuring 250.
SNHR called on the United Nations Human Rights Council to exert pressure on the Syrian government to end its use of the internationally prohibited weapons and to hold allies of the government such as Russia, Iran and China responsible for Syria's "excesses."
The Security Council should use satellites to monitor Syrian chemical weapons warehouses, SNHR recommended, while the Arab League should provide protective equipment and medical materials to local councils and families exposed to chemical weapons.